Radio base station, mobile communication system, handover control method, and program

ABSTRACT

A radio base station that performs radio communication with a mobile station in a cell and that includes: resource management unit that secures resources necessary for radio communication with the mobile station; and communication control unit that, in accordance with a reconnection request from the mobile station, uses the resources secured by the resource management unit to perform reconnection control for establishing radio communication with the mobile station. If a reconnection request is received from the mobile station that takes another radio base station as the connection source in a state in which resources have not been secured by resource management unit, communication control unit interrupts the reconnection control, executes handover with the other base station in order to switch the connection destination of the mobile station from the other base station to its own base station, causes resource management unit to secure resources, and then resumes the reconnection control.

TECHNICAL FIELD

The present invention relates to the technology of handover of a mobilecommunication system.

BACKGROUND ART

As disclosed in 3GPP TS 36.300, in an LTE (Long Term Evolution) mobilecommunication system, in order to realize mobility of a user whilemaintaining the communication quality of a mobile station (UE), the UEcan be handed over from the original radio base station to a radio basestation having a stronger received signal level.

When handover fails for some reason or when a problem occurs in theradio link during communication, communication can be recovered bycarrying out reconnection control of the RRC (Radio Resource Control)connection between the UE and the radio base station.

In an LTE system, handover is arranged by the following procedures.First, a handover request is transmitted from the handover source radiobase station (Source eNB) to the handover destination radio base station(Target eNB), and the Target eNB that has received the handover requestjudges whether the UE is able to obtain access based on access controlrules such as service authorization and load. When it is determined thatthe UE is able to obtain access, the Target eNB then secures resourcesfor the UE and returns to the Source eNB a signal indicating that thehandover request will be accepted.

Handover is realized by means of the above-described arrangements. Ifhandover fails for some reason, service can be continued byre-connecting the RRC connection and again attempting handover.

On the other hand, in order that reconnection control of the RRCconnection may succeed, context information of the UE that includes acell identifier that identifies the cell in which communication wasrealized up to this point, a UE identifier that uniquely identifies theUE in that cell, and an authentication code must be saved beforehand inthe radio base station in which reconnection is performed. During theabove-described handover preparation procedure, reconnection control canbe implemented by transmitting the UE context information to the TargeteNB.

In the above-described LTE system, techniques have been proposed forraising the success rate of handover or reconnection following handoverfailure.

For example, in Patent Document 1, the radio base station of the originof movement selects a plurality of cells having good communicationquality on the basis of communication quality reports of neighboringcells from the UE and implements a handover preparation procedure withrespect to the plurality of radio base stations that manage these cells.The radio base station of the origin of movement then transmitsinformation of the plurality of radio base stations that have completedthe handover preparation to the UE.

The UE selects the movement destination cell that performs handoverbased on information of the radio base stations that have completed thehandover preparation from the radio base station of the origin ofmovement and the communication quality of the neighboring cells at thetime of receiving the information.

Literature of the Prior Art Patent Documents

-   Patent Document 1: WO2009/142054

SUMMARY OF THE INVENTION

In the method described in Patent Document 1, when there is a radio basestation having higher reception quality than the radio base stationsthat have been set as candidates of handover destination, the UEtransmits a reconnection request to that radio base station. When theradio base station receives the reconnection request in a state in whichthe radio resources, that are necessary for radio communication with theUE, have not been secured, the radio base station transmits a responseto the UE indicating that reconnection cannot be carried out. As aresult, the UE is unable to reconnect and is unable to receive thecommunication service. Reconnection thus fails in some cases.

It is therefore an object of the present invention to provide a radiobase station, a mobile communication system, a handover control method,and a program that can raise the success rate of reconnection at thetime of implementing handover and thus solve the above-describedproblem.

According to one aspect of the present invention for achieving theabove-described object, a radio base station is provided that isprovided with a cell for which an area, in which radio communication ispossible, is prescribed and that carries out radio communication with amobile station within that cell, the radio base station including:

a radio resource management unit that secures resources necessary forradio communication with the mobile station; and

a communication control unit that, in accordance with a reconnectionrequest from the mobile station that has executed handover for switchinga connection destination, uses the resources that were secured by theradio resource management unit to carry out reconnection control toestablish radio communication with the mobile station;

wherein the communication control unit, when the reconnection request isreceived from a mobile station that takes another radio base station asthe connection source in a state in which the resources have not beensecured by the radio resource management unit, interrupts thereconnection control, executes handover with the other radio basestation in order to switch the connection destination of the mobilestation to its own base station to cause the radio resource managementunit to secure the resources, and then resumes the reconnection control.

According to another aspect of the present invention, a mobilecommunication system is provided that includes:

a mobile station; and

a plurality of radio base stations that are each provided with a cellfor which an area, in which radio communication is possible, isprescribed and that each perform radio communication with the mobilestation within the cell;

wherein each of the plurality of radio base stations includes:

a radio resource management unit that secures resources necessary forradio communication with the mobile station; and

a communication control unit that, in accordance with a reconnectionrequest from the mobile station that has executed handover for switchingthe connection destination, uses the resources that have been secured bythe radio resource management unit to perform reconnection control toestablish radio communication with the mobile station; and

the communication control unit, upon receiving the reconnection requestfrom the mobile station that takes another radio base station as theconnection source in a state in which the resources have not beensecured by the radio resource management unit, interrupts thereconnection control, executes handover with the other radio basestation in order to switch the connection destination of the mobilestation to its own base station to cause the radio resource managementunit to secure the resources, and then resumes the reconnection control.

According to yet another aspect of the present invention, a handovercontrol method is provided in which a radio base station that isprovided with a cell for which an area in which radio communication ispossible is prescribed and that performs radio communication with amobile station in the cell:

secures resources that are necessary for radio communication with themobile station, and in accordance with a reconnection request from themobile station that has executed handover for switching the connectiondestination, uses the resources that were secured to performreconnection control to establish radio communication with the mobilestation; and

when the reconnection request is received from the mobile station thattakes another radio base station as the connection source in a state inwhich the resources have not been secured, interrupts the reconnectioncontrol, executes handover with the other radio base station in order toswitch the connection destination of the mobile station to its own basestation, and after securing the resources, resumes the reconnectioncontrol.

According to another aspect of the present invention, a program isprovided for causing a computer of a radio base station that is providedwith a cell for which an area, in which radio communication is possible,is prescribed and that performs radio communication with a mobilestation within the cell to execute processes of:

securing resources necessary for radio communication with the mobilestation, and, in accordance with a reconnection request from the mobilestation that has executed handover for switching the connectiondestination, uses the resources that were secured to performreconnection control to establish radio communication with the mobilestation; and

when the reconnection request is received from the mobile station thattakes another radio base station as the connection source in a state inwhich the resources have not been secured, interrupting the reconnectioncontrol, executing handover with the other radio base station in orderto switch the connection destination of the mobile station to its ownbase station, and after having secured the resources, resuming thereconnection control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of the mobilecommunication system that is an exemplary embodiment of the presentinvention.

FIG. 2 is a block diagram showing the configuration of the principalparts of a mobile station of the mobile communication system shown inFIG. 1.

FIG. 3 is a block diagram showing the configuration of the principalparts of the radio base station of the mobile communication system shownin FIG. 3.

FIG. 4 is a flow chart showing an example of the operations of thehandover control unit of the mobile station shown in FIG. 2.

FIG. 5 is a flow chart showing the procedure of, from among theoperations that accompany handover of the radio base station shown inFIG. 3, operations of the handover source.

FIG. 6 is a flow chart showing the procedure of, from among theoperations that accompany handover of the radio base station shown inFIG. 3, operations of the handover destination.

FIG. 7 is a flow chart showing the procedure of, from among operationsthat accompany handover of the radio base station shown in FIG. 3,operations of the reconnection request destination.

FIG. 8 is a view for describing an example of reconnection notification.

FIG. 9 is a view for describing the series of mobile communicationprocesses that accompany handover when, from among the mobile station,the radio base station that is the handover source, the radio basestation that is the handover destination, and a radio base station otherthan the handover source/destination of the mobile communication systemshown in FIG. 1, a radio base station other than the handoversource/destination receives a reconnection request.

EXPLANATION OF REFERENCE NUMBERS

-   -   10 mobile station    -   11, 21 control unit    -   12, 22 handover control unit    -   13, 23 communication unit    -   14, 24 memory    -   20 radio base station    -   30 core network    -   221 resource management unit    -   222 communication control unit    -   231 radio communication unit    -   232 S1 interface unit    -   233 X2 interface unit

BEST MODE FOR CARRYING OUT THE INVENTION

An exemplary embodiment of the present invention is next described withreference to the accompanying drawings.

FIG. 1 is a block diagram showing the configuration of the mobilecommunication system that is an exemplary embodiment of the presentinvention.

The mobile communication system shown in FIG. 1 is a mobilecommunication system that adopts the LTE mode described in 3GPP TS36.300 and includes: mobile station 10, a plurality of radio basestations 20 that are each equipped with a cell, and that perform radiocommunication with mobile station 10 in the cell, and core network 30.

Core network 30 is provided with S-GW (Serving Gateway) that performsrouting of user packets and buffering processes and an MME (MobilityManagement Entity) that performs authentication control of ahigher-order level. Each radio base station 20 and S-GW/MME areconnected by S1 links. Radio base stations 20 are interconnected by X2links.

The transmission and reception of information between radio basestations 20 is carried out by using S1 links, X2 links, or both types oflinks.

In the example shown in FIG. 1, there is one mobile station 10 and threeradio base stations 20, but the present invention is not limited tothese numbers. An actual mobile communication system is made up of aplurality of mobile stations 10 and a plurality of radio base stations20.

FIG. 2 is a block diagram showing the configuration of the principalparts of mobile station 10.

Referring to FIG. 2, the principal parts of mobile station 10 includecontrol unit 11, communication unit 13, and memory 14.

Within a cell, communication unit 13 carries out radio communicationwith radio base station 20 that manages the cell. In the vicinity of theborder of the cell, communication unit 13 both receives radio signalsfrom radio base station 20, that manages the cell in which communicationis currently carried out, and receives radio signals from other radiobase stations 20 that manage adjacent cells.

Control unit 11 is made up of a CPU (Central Processing Unit) andcontrols the operations of mobile station 10. Control unit 11 includeshandover control unit 12 that performs processing that is necessary forhandover.

When carrying out handover, handover control unit 12 acquires reportingconditions from radio base station 20 that is the handover source thatmanages the cell currently engaged in communication and saves thisinformation. Here, reporting conditions include conditions for judgingwhether there are adjacent cells in which the reception quality ishigher than the cell of the handover source, the difference being equalto or greater than a threshold value, and if there is more than oneadjacent cell that satisfy the judgment conditions, then the informationof these adjacent cells is reported.

For example, when handover control unit 12 measures the receptionquality of radio signals that are received in communication unit 13 ateach fixed time interval and when there is an adjacent cell thatsatisfies the reporting conditions, handover control unit 12 thentransmits to radio base station 20, that is the handover source, a listthat includes identification information of the adjacent cell and themeasurement result of the reception quality.

Handover control unit 12 further receives handover instructions fromradio base station 20 that is the handover source by way ofcommunication unit 13, and when handover to a radio base station that isthe handover destination fails due to the deterioration of radio qualityor due to failure of random access to the radio base station that is thehandover destination, measures the reception quality and selects thecell which has the highest reception quality.

Handover control unit 12 further transmits a reconnection request toradio base station 20 that is managing the cell that was selected duringcell reselection and performs the reconnection procedure andreconfiguration procedure with this radio base station 20.

A reconnection request contains a cell identifier of the cell of theradio base station that is the connection source of mobile station 10(the cell that is up for reconnection), a mobile station identifier thatuniquely identifies mobile station 10 that is in that cell, and theauthentication code of mobile station 10.

Here, the cell that has entered a state requiring reconnection is thehandover source cell or the handover destination cell. The handoversource cell or handover destination cell is designated depending on thedegree of progress of handover when reconnection became necessary.

For example, if handover has not been completed and the connectiondestination of mobile station 10 is the handover source radio basestation, the handover source cell is designated as the cell that hasentered a state requiring reconnection. On the other hand, if handoveris substantially completed and the connection destination of mobilestation 10 is the radio base station that is the handover destination,the cell that is the handover destination is designated as the cell thathas entered a state requiring reconnection.

Memory 14 is a storage means which is representative of semiconductormemory and in which programs or data that are needed for operatingmobile station 10 are stored. For example, programs or data that areneeded to execute processing necessary for carrying out handover arestored in memory 14, and the functions of, for instance, handovercontrol unit 12 can be provided by a CPU that executes programs that arestored in memory 14.

FIG. 3 is a block diagram showing the configuration of the principalparts of radio base station 20.

Referring to FIG. 3, the principal parts of radio base station 20include control unit 21, communication unit 23, and memory 24.

Communication unit 23 includes radio communication unit 231, S1interface unit 232, and X2 interface unit 233.

Radio communication unit 231 performs radio communication with mobilestation 10 that is in the cell of its own station. S1 interface unit 232communicates with S-GW/MME by way of S1 links. X2 interface unit 233communicates with other radio base stations by way of X2 links.

Control unit 21 is made up of a CPU and includes handover control unit22 that executes processing that is necessary for handover.

Handover control unit 22 includes resource management unit 221 thatsecures the resources necessary for radio communication with mobilestation 10, and communication control unit 222 that, in accordance withreconnection requests from mobile station 10 that has executed handoverfor switching the connection destination, uses the resources secured byresource management unit 221 to carry out reconnection control forestablishing radio communication with mobile station 10.

Communication control unit 222, upon receiving a reconnection requestfrom mobile station 10 that takes another radio base station as theconnection source in a state in which resources of mobile station 10have not been secured in resource management unit 221, interruptsreconnection control, executes handover with the other radio basestation for switching the connection destination of mobile station 10 toits own base station to cause resource management unit 221 to secureresources of mobile station 10, and then resumes reconnection control.

Memory 24 is storage means, concerning which a semiconductor memory isrepresentative, and stores programs and data necessary for operatingradio base station 20. For example, programs or data that are needed toexecute processing necessary for carrying out handover are stored inmemory 24, and the functions of, for instance, handover control unit 22can be provided by a CPU that executes programs that are stored inmemory 24.

In the above-described radio base station 20, an adjacent base stationtable, in which base station identifiers that identify the adjacentradio base stations and cell identifiers that identify the adjacentcells that are managed by these adjacent radio base stations are storedin association with each other for each of a plurality of adjacent radiobase stations that are adjacent to its own base station, may be storedin memory 24. In this case, communication control unit 222 may receivefrom mobile station 10 a reconnection request that includes a cellidentifier that identifies the cell of the radio base station that isthe connection source (the cell that is up for reconnection), a mobilestation identifier that uniquely identifies mobile station 10 withinthat cell, and the authentication code of mobile station 10, acquirefrom the adjacent base station table the base station identifier thatcorresponds to the cell identifier of the reconnection request, andtransmit to the adjacent radio base station that is identified by thebase station identifier a reconnection notification that includes thecell identifier, the mobile station identifier, and the authenticationcode that are included in the reconnection request.

In addition, upon having received, from a second adjacent radio basestation during execution of handover for switching the connectiondestination of mobile station 10 from a first adjacent radio basestation to its own base station, a reconnection notification thatincludes a cell identifier that identifies the cell (the cell that is upfor reconnection) of the radio base station that is the connectionsource of mobile station 10, the mobile station identifier that uniquelyidentifies mobile station 10 within the cell, and the authenticationcode of mobile station 10, communication control unit 222 may bothtransfer the reconnection notification to the first adjacent radio basestation and release the resources that were secured in resourcemanagement unit 221.

Further, communication control unit 222, when having received, from afirst adjacent radio base station and second adjacent radio basestation, a reconnection notification that contains the cell identifierof the cell of the radio base station that is the connection source ofmobile station 10 (the cell that is up for reconnection), the mobilestation identifier that uniquely identifies mobile station 10 within thecell, and the authentication code of mobile station 10 when executinghandover for switching the connection destination of mobile station 10from its own station to the first adjacent radio base station, mayexecute handover with the adjacent radio base station from which thereconnection notification was first received for switching theconnection destination of mobile station 10 from its own station to theadjacent radio base station.

Further, when communication control unit 222 receives reconnectionnotification from the first adjacent radio base station after havingreceived reconnection notification from the second adjacent radio basestation, communication control unit 222 may instruct the first adjacentradio base station to release the resources of mobile station 10.

The operations of the mobile communication system of the presentexemplary embodiment are next described more concretely.

The operations that accompany handover of mobile station 10 are firstdescribed.

FIG. 4 is a flow chart showing an example of the operations of handovercontrol unit 12 of mobile station 10.

Referring to FIG. 4, handover control unit 12 carries out measurement ofreception quality in Step S10. In Step S11, handover control unit 12judges whether the measurement result of reception quality meets thereporting conditions. If the reporting conditions are not satisfied, thehandover process is ended.

If the reporting conditions are satisfied, handover control unit 12creates a list that contains information of adjacent cells in whichreception quality is higher than the cell that is the movement sourceand in which this difference is equal to or greater than a thresholdvalue and transmits this adjacent cell list to radio base station 20that is the handover source (Source eNB). The adjacent cell listcontains the cell identifiers of the adjacent cells and the base stationidentifiers of radio base stations 20 that manage the adjacent cells.

In Step S13, handover control unit 12 next judges whether a handoverinstruction has been received from radio base station 20 that is thehandover source (Source eNB).

Next, in Step S14, when a handover instruction is received and thehandover to the radio base station that is the handover destinationfails (when performing cell reselection) due to the deterioration ofreception quality or due to a failure of random access to the radio basestation that is the handover destination, handover control unit 12selects the cell having the highest reception quality.

In Step S15, handover control unit 12 next transmits a reconnectionrequest to radio base station 20 that is managing the adjacent cell thatwas selected in the cell reselection.

Finally, in Step S16, handover control unit 12 executes a reconnectionprocedure and a reconfiguration procedure with radio base station 20that is the reconnection request destination.

The operations that accompany handover of radio base station 20 are nextdescribed.

The operations that accompany this handover of radio base station 20include a first operation as the handover source, a second operation asthe handover destination, and a third operation as the reconnectionrequest destination.

FIG. 5 is a flow chart showing the procedure of the first operation asthe handover source.

Referring to FIG. 5, in Step S20, communication control unit 222receives a communication quality report that includes an adjacent celllist from mobile station 10 by way of radio communication unit 231.

In Step S21, communication control unit 222 judges whether handover isnecessary based on, for example, the adjacent cell list. In judging thenecessity of handover, communication control unit 222 judges thenecessity of handover based on conditions of whether access isrestricted due to large CPU processing load or based on conditions ofwhether service is provided to mobile station 10 for each adjacent cellin the adjacent cell list.

In Step S22, communication control unit 222 excludes from the adjacentcell list cells in which access is restricted or cells in which serviceis not provided to mobile station 10 and determines, from among theremaining cells, the cell having the highest reception quality as thehandover destination cell.

In the above-described Step S22, communication control unit 222 checkswhether the handover destination cell that was determined is a cellunder the jurisdiction of its own base station.

If the cell is not under its own jurisdiction, communication controlunit 222 in Step S23 transmits a handover preparation request to theradio base station (Target eNB) that is managing the handoverdestination cell.

The handover preparation request may contain the context information ofmobile station 10, for example, a service identifier for identifying theservice being provided to the user by the radio base station that is thehandover source, setting information of the radio resources forproviding the service, setting information of a security function, acell identifier of the handover source cell, a mobile station identifierfor uniquely identifying mobile station 10 within the cell, and theauthentication code of mobile station 10.

The transmission of the handover preparation request is implemented byway of S1 interface unit 232 or X2 interface unit 233. For example,communication control unit 222 may check whether communication has beenestablished between its own base station and the Target eNB by means ofan X2 link, and if communication has been established, may transmit thehandover preparation request by X2 interface unit 233.

The link used for the transmission of the handover preparation requestmay be designated in advance. Alternatively, when communication has beenestablished between its own base station and the Target eNB by way ofeach of an S1 link and an X2 link, communication control unit 222 maypreferentially use an X2 link to transmit the handover preparationrequest.

In Step S24, communication control unit 222 judges whether a response tothe handover preparation request has been received by way of S1interface unit 232 or X2 interface unit 233. This handover preparationrequest response contains information indicating whether handover ispossible and a handover instruction that includes a cell identifier ofthe handover destination cell and information of the radio resources ofmobile station 10.

In Step S25, communication control unit 222 judges whether the handoverpreparation request response indicates that handover is possible. Ifhandover is not possible, the handover process is ended.

If handover is possible, communication control unit 222 activates an HOcompletion notification waiting timer and transmits the handoverinstruction contained in the handover preparation request response tomobile station 10 by means of radio communication unit 231 in Step S26.

If the handover destination cell that was determined is under thejurisdiction of its own base station in the above-described Step S22,handover is executed between the cells of its own base station based onthe above-described Steps S23-S26.

In Step S27, communication control unit 222 checks whether or not the HOcompletion notification waiting timer has timed out. If the timer hastimed out, the handover process is ended.

In Step S28, communication control unit 222 judges whether areconnection notification was received before the HO completionnotification waiting timer timed out. The reconnection notificationincludes the cell identifier of the cell (the cell that is up forreconnection) of the connection source radio base station, the mobilestation identifier of the mobile station in the cell, the authenticationcode of the mobile station, and the cell identifier of the cell (thereconnection request destination cell) of the radio base station thatreceived the reconnection request.

If a reconnection notification has been received, communication controlunit 222 checks the UE context in Step S29. More specifically,communication control unit 222 checks whether the cell identifier,mobile station identifier, and authentication code contained in thereconnection notification are held in its own base station. If they arenot held in its own base station, the radio base station operates as theradio base station of the reconnection request destination (Non-preparedeNB) (refer to the third operation, to be described).

If the information is held, communication control unit 222 halts the HOcompletion notification waiting timer in Step S30.

In Step S31, communication control unit 222 identifies the radio basestation that manages the reconnection request destination cell based onthe cell identifier of the reconnection request destination cell that iscontained in the reconnection notification. In addition, communicationcontrol unit 222 issues a request to the handover destination radio basestation to release the radio resources that were secured in the handoverpreparation procedure.

In Step S32, communication control unit 222 transmits a handoverpreparation request to the radio base station that was identified bymeans of S1 interface unit 232 or X2 interface unit 233.

In Step S33, communication control unit 222 judges whether a response tothe handover preparation request has been received by way of S1interface unit 232 or X2 interface unit 233. This handover preparationrequest response includes information indicating whether handover ispossible and a handover instruction that contains the cell identifier ofthe handover destination cell (in this case, identical to thereconnection request destination) or information of the radio resourcesof mobile station 10.

In Step S34, communication control unit 222 judges whether the handoverpreparation request response indicates that handover is possible. Ifhandover is not possible, the handover process is ended.

If handover is possible, communication control unit 222 activates the HOcompletion notification waiting timer in Step S35.

In Step S36, communication control unit 222 checks whether the HOcompletion notification waiting timer has timed out. If the timer hastimed out, the handover process is ended.

In Step S37, communication control unit 222 judges whether a handovercompletion notification was received by way of S1 interface unit 232 orX2 interface unit 233 before the HO completion notification waitingtimer timed out.

If the handover completion notification has been received, communicationcontrol unit 222 halts the HO completion notification waiting timer andreleases the UE context in Step S38.

FIG. 6 is a flow chart showing the procedure of the second operation asthe handover destination.

Referring to FIG. 6, in Step S40, communication control unit 222 judgeswhether a handover preparation request has been received from thehandover source radio base station by way of S1 interface unit 232 or X2interface unit 233.

If a handover preparation request has been received, communicationcontrol unit 222 judges whether the handover preparation request can beaccepted in Step S41. If the handover preparation request can beaccepted, communication control unit 222 saves the context informationof mobile station 10 in memory 24, secures the radio resources of itsown base station that are necessary for providing service to mobilestation 10, and compiles a handover instruction that containsinformation of the cell identifier of the handover destination cell andthe radio resources that are set in mobile station 10.

In Step S42, communication control unit 222 transmits a handoverpreparation request response, that contains the above-described handoverinstruction that was compiled, to the handover source radio base stationby means of S1 interface unit 232 or X2 interface unit 233. If thehandover preparation request cannot be accepted, communication controlunit 222 transmits a response indicating that the handover preparationrequest cannot be accepted to the handover source radio base station.

In Step S43, communication control unit 222 activates waiting timer foraccess from mobile station 10 that corresponds to the handover request.The timing of the timing-out of this timer is the trigger for releasingthe context information that was saved and the radio resources that weresecured in Step S41.

In Steps S44 and S45, communication control unit 222 judges whether areconnection notification is received before the timer times out. Here,the reconnection notification includes a cell identifier of the cell(the cell that is up for reconnection) of the connection source radiobase station, the mobile station identifier of the mobile station in thecell, the mobile station authentication code, and the cell identifier ofthe cell (cell to which reconnection is requested) that received thereconnection request.

When the reconnection notification is received, communication controlunit 222 checks the UE context information in Step S46. In Step S47,communication control unit 222 then judges whether its own base stationis the handover destination (target). More specifically, communicationcontrol unit 222 checks whether the cell identifier, mobile stationidentifier, and authentication code that are contained in thereconnection notification are held in its own base station, and if held,determines that its own base station is the handover destination(target).

If the cell identifier, mobile station identifier and authenticationcode that are contained in the reconnection notification are not held inits own base station, the radio base station operates as the radio basestation that is a reconnection request destination (Non-prepared eNB)(refer to the third operation described hereinbelow).

If its own base station is the handover destination (target),communication control unit 222 in Step S48 halts the timer and releasescontext information that was saved and the radio resources that weresecured in Step S41.

If the timer has timed out in Step S44 described hereinabove,communication control unit 222 in Step S49 releases the contextinformation that was saved and the radio resources that were secured inStep S41.

FIG. 7 is a flow chart showing the procedure of the third operation asthe reconnection request destination.

Referring to FIG. 7, communication control unit 222 in Step S50 judgeswhether a reconnection request has been received. A reconnection requesthere includes the cell identifier of the cell (the cell that is up forreconnection) of the connection source radio base station of mobilestation 10, the mobile station identifier that uniquely identifiesmobile station 10 in the cell, and the authentication code of mobilestation 10.

If a reconnection request has been received, communication control unit222 in Step S51 starts reconnection control in accordance with thereconnection request, and checks the UE context information. Morespecifically, communication control unit 222 checks whether the cellidentifier of the cell (the cell that is up for reconnection) of theconnection source radio base station, the mobile station identifier, andauthentication code that are contained in the reconnection request havebeen saved in its own station. If this information has not saved,communication control unit 222 executes the processing of Step S52 andsubsequent processes. If the information has been saved, the radio basestation operates as a radio base station that is the handover source orhandover destination.

In Step S52, communication control unit 222 interrupts the reconnectioncontrol and activates the handover preparation request waiting timer.

In Step S53, communication control unit 222 acquires from the adjacentbase station table the base station identifier that corresponds to thecell identifier that is contained in the reconnection request andtransmits by means of S1 interface unit 232 or X2 interface unit 233 areconnection notification to the adjacent radio base station that isspecified by the base station identifier.

The reconnection notification contains, for example, informationelements such as shown in FIG. 8. In the example of FIG. 8, theinformation elements include: the identifier “Message Type” thatindicates a reconnection notification, the cell identifier “Failure CellIdentity” of the cell that is up for reconnection, the UE identifier “UEIdentity” of the cell that is up for reconnection, the authenticationcode “Authentication Code” of the UE, and the cell identifier“Re-establishment Cell Identity” of the cell in which the reconnectionrequest was received.

In Steps S54 and S55, communication control unit 222 judges whether ahandover preparation request was received from the handover source radiobase station by way of S1 interface unit 232 or X2 interface unit 233before time-out. If the timer timed out, communication control unit 222ends the handover process.

If a handover preparation request was received, communication controlunit 222 in Step S56 judges whether the handover preparation request canbe accepted. If the handover preparation request can be accepted,communication control unit 222 saves the context information of mobilestation 10 in memory 24, secures the radio resources of its own stationthat are necessary for providing service to mobile station 10, and haltsthe handover preparation request waiting timer. Communication controlunit 222 then transmits a response indicating that the handoverpreparation request has been accepted to the handover source radio basestation by way of S1 interface unit 232 or X2 interface unit 233.

In Step S57, communication control unit 222 resumes reconnectioncontrol. In Step S58, communication control unit 222 carries out thereconnection procedure and the reconfiguration procedure with thehandover source radio base station.

In Step S59, communication control unit 222 transmits a path-switchinginstruction to core network 30 by way of S1 interface unit 232.

In Step S60, communication control unit 222 transmits a handovercompletion notification to the handover source radio base station by wayof S1 interface unit 232 or X2 interface unit 233.

Next, as the specific operations of the mobile communication system ofthe present exemplary embodiment, a series of mobile communicationprocesses that accompany handover is described for a case in which, fromamong a mobile station (UE), a handover source radio base station(Source eNB), a handover destination radio base station (Target eNB),and a radio base station other than the handover source/destination(Non-prepared eNB), the radio base station other than the handoversource/destination receives a reconnection request.

FIG. 9 is a view for describing the procedure of such a series of mobilecommunication processes that accompany handover.

Referring to FIG. 9, in Step S201, the UE measures reception quality ofadjacent cells, and when the reporting conditions instructed from theSource eNB that manages the cell in which communication is carried outare satisfied, transmits to the Source eNB the reception quality of theplurality of adjacent cells obtained in the measurement.

In Step S202, the Source eNB judges whether handover needs to beperformed. If the current state satisfies the handover triggerconditions, the Source eNB judges that the UE requires handover andexecutes Step S203. If the current state does not satisfy the handovertrigger conditions, the Source eNB ends the mobile communicationprocessing.

In Step S203, the Source eNB determines the cell having the highestreception quality value to be the handover destination cell, andtransmits a handover preparation request to the Target eNB that ismanaging this cell.

In the above-described Step S203, the Source eNB is able to excludecells in which load is great or to which access is restricted and toexclude cells that are not providing service to the UE and is then ableto determine from among the remaining cells the cell having the highestreception quality value as the handover destination cell.

In addition, the Source eNB is able to both check whether the handoverdestination cell is a cell under the control of its own eNB and whetheran X2 interface link has been established with the radio base stationthat manages the handover destination cell, and further, to determinewhether to use the X2 interface or the S1 interface for transmitting thehandover preparation request.

Still further, the Source eNB may add context information of the UE (forexample, an identifier of service that was offered to the user in theSource eNB, setting information of the radio resources for providing theservice, setting information of a security function, the identifier ofthe handover source cell, a UE identifier for uniquely identifying theUE within the cell, and an authentication code) to the handoverpreparation request. Thus, in subsequent steps, the Target eNB will beable to determine the required resources of the UE based on this contextinformation and, even following the failure of handover due to somereason and the subsequent implementation of reconnection of the RRCconnection, the Target eNB will be able to reattempt handover andcontinue providing service.

In Step S204, the Target eNB that has received the handover preparationrequest judges whether the handover preparation request can be accepted.

If the handover preparation request can be accepted, the Target eNBsaves the context information of the UE, secures resources of its owneNB that are necessary for providing service to the UE, and compiles ahandover instruction that contains the identifier of the handoverdestination cell and information of the radio resources that are set tothe UE. The Target eNB then activates a timer for waiting access fromthe UE that corresponds to the handover request that serves as thetrigger for releasing the context information of the UE and theresources secured for the UE.

In Step S205, the Target eNB transmits to the Source eNB a responseindicating acceptance of the handover preparation request and containingthe previously compiled handover instruction. If the handoverpreparation request cannot be accepted, the Target eNB transmits to theSource eNB a response indicating that the handover cannot be accepted.

In Step S206, the Source eNB that has received the handover preparationrequest response activates a waiting timer for a handover completionnotification from the Target eNB that corresponds to the handoverrequest, this notification serving as the trigger for releasing thecontext information of a UE for which handover has succeeded. Ifcommunication data that have been transmitted from the UE are beingbuffered or if communication data that the UE are to receive are beingbuffered, the Source eNB can transmit the communication data that arebeing buffered to the Target eNB. After an RRC connection has beensuccessfully established between the UE and the Target eNB, thecommunication data are transferred to a higher-order S-GW or aretransmitted to the UE by the Target eNB.

In Step S207, the Source eNB transmits the handover instruction that wascontained in the handover preparation request response to the UE, andinstructs the UE to access the Target eNB.

In Step S208, if the UE encounters a failure in accessing the Target eNBdue to, for example, deterioration of the radio quality or a failure ofrandom access to the Target eNB, an appropriate cell is selected torecover the handover as quickly as possible and continue the servicebeing received by the UE.

In Step S209, the UE transmits to the radio base station that managesthe selected cell a reconnection request that includes the identifier ofthe cell that is up for reconnection, the UE identifier for uniquelyidentifying the UE within the cell, and the authentication code.

In Step S210, the radio base station that has received the reconnectionrequest checks whether the identifier of the cell that is up forreconnection, the UE identifier of the cell, and the authentication codethat were contained in the reconnection request have been saved in thehandover preparation procedure. If the information has been saved, theradio base station judges that its own station is the Target eNB, haltsthe access waiting timer, and continues the reconnection procedure,thereby succeeding in recovering from the handover failure.

If the information has not been saved, the radio base station in StepS211 judges that its own base station is a Non-Prepared eNB, and at thepoint of originally rejecting the reconnection request because thecontext information of the UE has not been saved, interrupts thereconnection control and identifies the radio base station that managesthe cell based on the identifier of the cell that is up for reconnectionand the adjacent base station table. The radio base station thenactivates the handover preparation request waiting timer that serves asthe trigger for resuming the interrupted reconnection control.

The adjacent base station table is made up of combinations of theidentifiers of cells adjacent to the Non-Prepared eNB and information ofthe radio base stations that manage the cells. The adjacent base stationtable may be prepared in advance as station data of the Non-PreparedeNB, or may be produced based on the information of cells that isexchanged among radio base stations by way of X2 links.

After activation of the handover preparation request waiting timer, theNon-Prepared eNB executes Step S212, Step S212′, or both steps.

In Step S212, the Non-Prepared eNB transmits a reconnection notificationto the Target eNB. In Step S212′, the Non-Prepared eNB transmits areconnection notification to the Source eNB. Here, a reconnectionnotification is made up of information elements such as shown in FIG. 8.

When transmitting a reconnection notification, the Non-Prepared eNB isable to check whether an X2 link has been established with the radiobase station that manages the cell that is up for reconnection (theSource eNB or Target eNB) and to determine whether to use the X2interface or the S1 interface for transmitting the reconnectionnotification.

After executing Step S212, in Step S213, the Target eNB that receivedthe reconnection notification checks whether the identifier of the cellthat is up for reconnection, the UE identifier of the cell, and theauthentication code that are contained in the reconnection notificationhave been saved.

As a result of the check described above, the Target eNB ignores thereconnection notification that was received if the information has notbeen saved.

If the above-described check shows that the information has been saved,the Target eNB in Step S214 checks whether the timer for awaiting theaccess from the reconnecting UE has been activated, and if the timer hasbeen activated, determines that its own base station is operating as theTarget eNB, halts the timer, and releases the context information of theUE and the resources that were secured for providing service to the UE.Then, in Step S215, the Target eNB transfers the reconnectionnotification to the Source eNB.

In Step S213′, after the execution of Step S212′ or after the executionof Step S215, the Source eNB that received the reconnection notificationchecks whether the identifier of the cell that is up for reconnection,the UE identifier of that cell, and the authentication code that arecontained in the reconnection notification have been saved.

If the above-described check shows that the information has not beensaved, the Source eNB ignores the reconnection notification that wasreceived.

If the above-described check shows that the information has been saved,the Source eNB in Step S216 checks whether the handover completionnotification waiting timer has been activated, and if the timer has beenactivated, determines that its own base station was operating as theSource eNB and halts the timer.

In Step S217, after halting the handover completion notification waitingtimer, the Source eNB identifies the Non-prepared eNB that manages thecell based on the identifier of the cell that received the reconnectionrequest that is contained in the reconnection notification and theadjacent base station table. When the radio base station thattransmitted the reconnection notification is not the Target eNB, theSource eNB instructs the Target eNB to release the resources that weresecured for the UE.

In Step S218, the Source eNB transmits a handover preparation request tothe Non-prepared eNB. Here, the Source eNB may also append the contextinformation of the UE (for example, the identifier of service that wasbeing provided to the user in the Source eNB, setting information of theradio resources for providing the service, setting information ofsecurity functions, the identifier of the handover source cell, the UEidentifier for uniquely identifying the UE within the cell, and theauthentication code) to the handover preparation request. In this way,the Non-prepared eNB in succeeding steps is able to, based on thiscontext information, determine the necessary resources of the UE or,even in the event that there is a reconnection control failure for somereason, after reconnection of the RRC connection, and re-attemptreconnection control to continue the service.

In Step S219, the Non-prepared eNB that received the handoverpreparation request saves the context information of the UE, secures theresources of its own eNB that are necessary for providing service to theUE, and halts the handover preparation request waiting timer.

In Step S220, the Non-prepared eNB transmits to the Source eNB aresponse indicating acceptance of the handover preparation request.

In Step S221, the Source eNB that has received the handover preparationrequest response activates the waiting timer for the handover completionnotification from the Non-prepared eNB that corresponds to the handoverrequest and that serves as the trigger for releasing the contextinformation of the UE for which handover has succeeded.

In the processing of the above-described Step S221, if the communicationdata that have been transmitted from the UE are being buffered or if thecommunication data that the UE are to receive are being buffered, theSource eNB is able to transfer communication data that are beingbuffered to the Non-prepared eNB. After the RRC connection between theUE and the Non-prepared eNB has been successfully reconnected, thesecommunication data are transferred to a higher-order S-GW or transmittedto the UE by the Non-prepared eNB.

In Step S222, the Non-prepared eNB executes a reconnection procedure anda Reconfiguration procedure of the RRC connection with the UE.

In Step S223, the Non-prepared eNB instructs the core network to switchthe connection to the UE provided between Source eNB and the corenetwork to the connection between the Non-prepared eNB and core network30. In Step S224, core network 30 reports the handover completion to theSource eNB.

In Step S225, the Source eNB that has received the handover completionnotification halts the handover completion notification waiting timer ofthe UE and releases the context information of the UE.

The mobile communication system of the present exemplary embodimentdescribed hereinabove exhibits the following action and effects.

In the method described in Patent Document 1, the UE must wait forresponses from the plurality of radio base stations that are candidatesas handover destinations, resulting in the lengthening of the timerequired for handover. In addition, because the plurality of radio basestations must secure resources for the same UE, the overall capacity ofthe system is reduced. Still further, of the radio base stations thatare taken as candidates, radio base stations other than the radio basestation that is selected by the UE must implement the handover cancelprocess, thereby increasing the processing load.

The mobile communication system of the present exemplary embodimenteliminates the need for the plurality of radio base stations toimplement the handover preparation procedure, whereby the success rateof reconnection at the time of executing handover can be improved.

In addition, the time and processing load required for handover can bereduced, and the pointless securing of resources can be limited.

Still further, when a radio base station has received a reconnectionrequest from a UE and has not secured radio resources of the UE in itsown base station, the radio base station, by interrupting reconnectioncontrol and implementing handover with the radio base station thatmanages the cell that is up for reconnection (the Source eNB or theTarget eNB), secures the radio resources of the UE and then resumes thereconnection control. In this way, reconnection control can beimplemented reliably and the success rate of reconnection when executinghandover can be further improved.

The mobile communication system of the present exemplary embodimentdescribed hereinabove is only one example of the present invention, andthe configuration is open to various modifications within a scope thatwill be clear to one of ordinary skill in the art based on the spirit ofthe present invention.

For example, in the mobile communication processing procedure of FIG. 9,upon being informed that handover cannot be accepted as the handoverpreparation request response of Step S205, the Source eNB may wait forthe next report of reception quality, or may determine, from among thecandidates of radio base stations that are capable of handover in thelist obtained in the report of Step S201, the radio base station thathas the next highest reception quality as the handover destination andexecute the process of Step S203. In the former case, the wastefulsecuring of resources can be limited compared to the latter case.

The program executed by the CPU of radio base station 20 (for example, aprogram for providing the capabilities of handover control unit 22) maybe provided on a recording medium that can be read by a computer, forexample, an optical disk such as a CD (Compact Disc) or DVD (DigitalVideo Disc) or by means of a USB (Universal Serial Bus) memory or memorycard, or may be provided by way of a communication network (for example,the Internet).

Although the present invention has been described with reference to anexemplary embodiment, the present invention is not limited to theabove-described exemplary embodiment. The configuration and operationsof the present invention are open to various modifications within ascope that does not depart from the spirit of the present invention thatwill be clear to one of ordinary skill in the art.

This application claims the benefits of priority based on JapanesePatent Application No. 2012-159637 for which application was submittedon Jul. 18, 2012 and incorporates by citation all of the disclosures ofthat application.

1. A radio base station that is provided with a cell for which an area,in which radio communication is possible, is prescribed and that carriesout radio communication with a mobile station within that cell,comprising: a radio resource management unit that secures resourcesnecessary for radio communication with said mobile station; and acommunication control unit that, in accordance with a reconnectionrequest from said mobile station that has executed handover forswitching a connection destination, uses said resources that weresecured by said radio resource management unit to carry out reconnectioncontrol to establish radio communication with said mobile station;wherein said communication control unit, when said reconnection requestis received from said mobile station that takes another radio basestation as the connection source in a state in which said resources havenot been secured by said radio resource management unit, interrupts saidreconnection control, executes handover with said other radio basestation in order to switch the connection destination of said mobilestation to its own base station to cause said radio resource managementunit to secure said resources, and then resumes said reconnectioncontrol.
 2. The radio base station as set forth in claim 1, furthercomprising: an adjacent base station table in which, for each of theplurality of adjacent radio base stations that are adjacent to that basestation, a base station identifier that identifies the adjacent radiobase station and a cell identifier that identifies an adjacent cell thatis managed by the adjacent radio base station are stored in associationwith each other, wherein said communication control unit receives fromsaid mobile station a reconnection request that contains a cellidentifier that identifies the cell of the connection source radio basestation, a mobile station identifier that uniquely identifies the mobilestation within the cell, and an authentication code of the mobilestation, acquires from said adjacent base station table the base stationidentifier that corresponds to said cell identifier of the reconnectionrequest, and transmits to the adjacent radio base station that isidentified by the base station identifier a reconnection notificationthat contains said cell identifier, mobile station identifier, andauthentication code.
 3. The radio base station as set forth in claim 2,wherein: said communication control unit, upon receiving a reconnectionnotification that contains a cell identifier that identifies the cell ofthe radio base station that is the connection source of said mobilestation, a mobile station identifier that uniquely identifies saidmobile station within the cell, and an authentication code of saidmobile station from a second adjacent radio base station duringexecution of handover for switching the connection destination of saidmobile station from a first adjacent radio base station to its own basestation, both transfers the reconnection notification to said firstadjacent radio base station and releases resources of said mobilestation that were secured in said radio resource management unit.
 4. Theradio base station as set forth in claim 2, wherein said communicationcontrol unit, upon receiving, from each of a first adjacent radio basestation and a second adjacent radio base station during execution ofhandover for switching the connection destination of said mobile stationfrom its own base station to said first adjacent radio base station, areconnection notification that contains a cell identifier thatidentifies the cell of the radio base station that is the connectionsource of said mobile station, a mobile station identifier that uniquelyidentifies said mobile station within the cell, and the authenticationcode of said mobile station, executes handover with the adjacent radiobase station concerning the reconnection notification that was receivedfirst in order to switch the connection destination of said mobilestation from its own base station to the adjacent radio base station. 5.The mobile station as set forth in claim 4, wherein said communicationcontrol unit, upon receiving said reconnection notification from saidfirst adjacent radio base station after said reconnection notificationfrom said second adjacent radio base station, instructs said firstadjacent radio base station to release resources of said mobile station.6. A mobile communication system comprising: a mobile station; and aplurality of radio base stations that are each provided with a cell forwhich an area, in which radio communication is possible, is prescribedand that each perform radio communication with said mobile stationwithin the cell; wherein each of said plurality of radio base stationscomprises: a radio resource management unit that secures resourcesnecessary for radio communication with said mobile station; and acommunication control unit that, in accordance with a reconnectionrequest from said mobile station that has executed handover forswitching the connection destination, uses said resources that have beensecured by said radio resource management unit to perform reconnectioncontrol to establish radio communication with said mobile station;wherein said communication control unit, upon receiving saidreconnection request from said mobile station that takes another radiobase station as the connection source in a state in which said resourceshave not been secured by said radio resource management unit, interruptssaid reconnection control, executes handover with said other radio basestation in order to switch the connection destination of said mobilestation to its own base station to cause said radio resource managementunit to secure said resources, and then resumes said reconnectioncontrol.
 7. A handover control method in which a radio base station thatis provided with a cell for which an area, in which radio communicationis possible, is prescribed and that performs radio communication with amobile station in the cell, said method comprising: said radio basestation securing resources that are necessary for radio communicationwith said mobile station, and in accordance with a reconnection requestfrom said mobile station that has executed handover for switching theconnection destination, using said resources that have been secured toperform reconnection control to establish radio communication with saidmobile station; and when said reconnection request is received from saidmobile station that takes another radio base station as the connectionsource in a state in which said resources have not been secured,interrupting said reconnection control, said radio base stationexecuting handover with said other radio base station in order to switchthe connection destination of said mobile station to its own basestation, and after securing the resources, said radio base stationresuming said reconnection control.
 8. A non-transitorycomputer-readable storage medium with a program used in a computer of aradio base station that is provided with a cell for which an area, inwhich radio communication is possible, is prescribed and that performsradio communication with a mobile station within the cell, said programbeing for causing the computer to execute processes of: securingresources necessary for radio communication with said mobile station,and, in accordance with a reconnection request from said mobile stationthat has executed handover for switching the connection destination,using said resources that were secured to perform reconnection controlto establish radio communication with said mobile station; and when saidreconnection request is received from said mobile station that takesanother radio base station as the connection source in a state in whichsaid resources have not been secured, interrupting said reconnectioncontrol, executing handover with said other radio base station in orderto switch the connection destination of said mobile station to its ownbase station, and after having secured said resources, resuming saidreconnection control.